1. Field of the Invention
This invention is in the field of fluorine chemistry and more particularly in the field of direct fluorination.
2. Description of the Prior Art
Although polymers have been known and manufactured for many years, it has only been recently that significant efforts have been initiated to provide functional groups on polymer surfaces. Such functional groups could provide new uses for the base polymers due to the nature of the functional group, or the functional group could provide a reactive site on the polymer surface so that a variety of chemical compounds could be reacted with the polymer.
One of the earlier applications for functionalized polymers involved the development of peptide syntheses based on chloromethylated polystyrene. See G. R. Stark, "Biochemical Aspects of Reactions on Solid Supports", Academic Press, New York (1971). These procedures avoided repetitious purifications in the syntheses of complicated peptides, and such techniques have become so successful that these "Merrifield resins" are now widely used and are commercially available. Chloromethylated polystyrene has also been used in more conventional organic synthesis work such as the promotion of intramolecular reactions without high dilution apparatus and the separation of polymer bound triphenylphosphine oxide after a Wittig reaction. See J. I. Crowley and H. Rapaport, J. Amer. Chem. Soc., 92, 6363 (1970); M. A. Draus and A. Patchornik, Israel J. Chem., 9, 269 (1971); S. V. McKinley and J. W. Rakshys, Chem. Comm., 134 (1972); and F. Camps, J. Castells, J. Font and F. Vela, Tetrahedron Lett., 1715 (1971).
Polystyrene has also been functionalized and subsequently used to form heterogeneous catalysts by reacting homogeneous catalysts with the functional groups. See R. H. Grubbs, C. Gibbons, L. C. Kroll, W. D. Bonds and C. H. Brubaker, J. Amer. Chem. Soc., 95, 2373 (1973); and, R. H. Grubbs, L. C. Kroll and E. M. Sweet, J. Macromol. Sci. Chem., A7, 1047 (1973). Heterogeneous catalysts have advantages over their homogeneous counterparts because they can be prepared in a more reactive form since dimerization reactions that occur in solution cannot occur with the bound species and because they are more easily recoverable.
In addition to the effort directed to the functionalization of polystyrene, some effort has been made to functionalize polyethylene. It has been recognized that polyethylene functionalized with carboxylic acid groups, for example, would be particularly useful because of the chemical and mechanical properties of polyethylene and because of the ease with which carboxylic acid units can be converted to esters, amides, ketones, etc. Several different oxidation procedures have been reported in the literature as successfully forming carboxylic acid groups on polyethylene surfaces. See B. G. Aristov, I. Yu. Babkin, F. K. Borisova, A. V. Kiselev and A. Ya. Korolev, Izv. Acad, Nauk, SSSR, Otd. Khim., 6, 1017 (1963); Akad, Nauk, SSSR, Bulletin, 927 (1963); F. H. Ancker and F. L. Baier, U.S. Pat. No. 3,556,955 (1971); and, R. L. Augustine, Ed., "Oxidation, Vol. I", Marcel Dekker, New York, 1969, p6.
Although most of the effort to functionalize polymers to date has been directed towards polystyrene and polyethylene, it would be desirable to be able to produce functionalized fluoropolymers. Fluoropolymers are known to exhibit outstanding high temperature properties and are also unusually chemically inert. Because of these properties, they are used in applications where severe environmental factors are encountered.
Functionalization of fluoropolymers has not heretofore been achieved, however, probably due to a number of factors. Fluoropolymers are extremely inert so that functionalizing the fully fluorinated polymer is unlikely to succeed. Another problem has been the difficulty in carrying out the fluorinations themselves. Whereas many compositions can be directly chlorinated or brominated, it has been recognized that fluorine is dissimilar to these halogens in regard to direct halogenation. See McBee et al., U.S. Pat. No. 2,533,132 and U.S. Pat. No. 2,614,129. In fact, even though direct fluorination is a highly desirable process, prior attempts to use direct fluorination have often produced low to mediocre yields. Additionally, the yields are known to decrease as the molecular complexity of reactants becomes greater, thereby making direct fluorination of polymers an even more difficult matter. It is stated in one literature article, for example, that the yield of required fluorocarbon decreases as the molecular complexity of a hydrocarbon precurser increases, and it is difficult to fluorinate hydrocarbons above C.sub.10 without extensive decomposition occuring. See R. E. Banks, "Fluorocarbons and Their Derivatives", Oldbourne Press, London, p. 7 (1964). It is even suggested in the patent literature that the treatment of polyfluoroalcohols with elementary fluorine results in destructive fragmentation of the carbon chain and loss of the functional group at the end of the chain. See Stallmann, U.S. Pat. No. 3,038,941.
Direct fluorination reactions involving elemental fluorine are characterized by quick evolution of large quantities of heat, ignition and flaming which promote product decomposition, often with explosive violence. The inablility to control direct fluorination reactions to produce high yields of the desired fluorinated reactant without concomitant fragmentation of the desired product has prevented direct fluorination from becoming a widely accepted method of fluorination. Because of these problems, a very diversified art has been developed to circumvent or obviate the use of fluorine gas by using inorganic, metallic fluorides, hydrogen fluoride, or electrolytic cells where no free fluorine is produced.
It has previously been believed to be extremely important to remove all sources of oxygen from direct fluorination systems. Free oxygen was thought, for example, to crosslink materials, presumably with epoxy bridges, which greatly decreased yield of the desired fluorinated product. It was also known to form carbonyl groups such as acyl fluorides and peroxides on contact with carbon radical sites.
Fluorine has been used to sensitize the oxidation of trichloro and tetrachloroethylene. See W. T. Miller, Jr. and A. L. Dittman, "The Mechanism of Fluorination. I. Fluorine Sensitized Oxidation of Trichloro and Tetrachloroethylene", Mechanism of Fluorination, vol. 78, p. 2793- (June 1956). In this work, elementary fluorine was added to excess oxygen and the mixture was passed into tetrachloroethylene and trichloroethylene to produce acid halides. This work was stated to provide convincing evidence that free radical initiation mechanisms controlled fluorine reactions and the authors concluded, based on this work, that the deleterious effect of oxygen upon the reactions of fluorine with organic compounds, even at low temperatures, was readily understandable.
More recently, work is described in the patent literature wherein certain polymers, such as polyethylene or polyesters, are treated to change their surface properties by subjecting them to fluorine gas of a mixture of fluorine and oxygen. See U.S. Pat. No 3,865,615 to Manly and Belgium patents 789,562 and 811,644. These processes are designed to modify only surface properties of the polymer to provide reactive sites where enzymes can attach, to increase the hydrophilicity, to increase the soil resistance, etc. Typically, very small percentages of fluorine are actually incorporated into the polymers in these reactions. Because of this, the bulk properties of the starting materials are not significantly altered.